In just a few years I will be celebrating 50 years since I was in the WT Borganism, but as I recall, no one was *required* to join the TMS, but it was considered a sign of "spiritual progress".
Like the Chinese Finger Trap, once in it is difficult to withdraw. The best strategy is to begin missing a bunch of assigned talks, which means that someone else will have to prepare a talk with only minutes notice. After a few of these stunts they will probably be happy to let you go.
As I recall, that was part of how I went from Pioneer/MS to zero in a brief time. I didn't want drama, I just wanted OUT.